The question's pretty self-explanatory. I've wanted to know this for a while.
First you must defeat the pokemon league. I don't think it matters which order its in know but collect all muscial props, complete the national dex, obtain all entralink powers and get a 49 streak in the gear station in both single battle and double
1=Beat the Elite 4
2=Get a 49 streak in the battle subway in both Super Single and Double
3=Obtain all entralink powers
4=Obtain all Pokemon musical props
5*=Complete the National Pokedex
By the way you don't have to do it in order
I do not entirely know, but I am pretty sure that the first level up is beating the elite four.